Circuit boards are important items of commerce, being used in virtually every electronic device. The “board” or supporting member of a circuit board and other electronic devices (such as the interposer in a flip-chip package) is an important component of such devices, and properties of the materials used to make such boards are important to the functioning of the electronic or electrical circuit. As electronic components have become more sophisticated, the demands placed upon the materials used for boards have increased. For example, for many applications it is preferred that the board have a coefficient of expansion which matches those of the chips mounted on the board, and/or that the board have a low dielectric constant and a low dissipation factor, especially when high frequency devices are mounted on the board. These three factors are often adversely affected by the absorption of moisture by the board materials, which changes the dimensions of the board and/or changes the dielectric constant and dissipation factor of the board itself, and/or causes warpage.
The simplest boards for relatively nondemanding applications are typically made from a thermoset resin such as an epoxy filled with a fibrous reinforcement such as glass fiber. The glass fiber, often in the form of a woven fabric, is saturated with liquid epoxy resin to form a “prepreg”, which is cured in the form of a board. As the demands on boards increase, the glass may be replaced by a higher modulus infusible fiber such as an aramid. However, fibers such as aramid fibers, and epoxy resins, absorb significant amounts of moisture, and so are sometimes unsuitable for use together in highly demanding circuit board uses. Thus, there is a need for improved circuit board materials having reduced moisture absorbance properties.
Japanese Patent Application 2000-334871 describes the preparation of a sheet from which a prepreg may be formed by “laminating” a three-layer structure in which the middle layer may be a nonwoven sheet containing synthetic organic fiber and the two outer layers may contain aramids or other infusible fibers. While this reference discloses that the two outer layers may contain synthetic organic fiber in an amount less than that contained in the inner layer, no mention is made that the laminated sheet contain a nonuniform distribution of synthetic organic fiber through the thickness of the laminated sheet.
Japanese Patent Application 11-117184 describes the preparation of a sheet from which a prepreg may be formed by forming a nonwoven sheet from aramid and LCP (liquid crystal polymers) fibers, heating the sheet under pressure to make the LCP flow, and then adding a thermoset resin to form a prepreg. Nothing is said about variation of the LCP concentration across the thickness of the sheet.
Japanese Patent Application 9-21089 describes the preparation of an LCP nonwoven sheet (paper) which is reported to have low water absorption. Other fibers can also be present in the sheet. The product, after being heated under pressure to partially consolidate the sheet, is apparently still a paper-like material.
Japanese Patent Application 11-229290 describes the preparation of a paper made from LCP and aramid fibers which can be impregnated with an epoxy resin which is then cured. The resulting board may be used as a circuit board. No melting or flow under heat and/or pressure of the LCP is described.